1. Field of the Invention
This invention relates to an actuator for an optical pick-up using the concentrated magnetization method that is used to drive an objective lens for converging a laser light beam onto an optical disc.
2. Description of the Related Art
Nowadays, there have been suggested various types of optical pick-up for recording and reproducing an information on and from an optical disc in accordance with the fast development of optical disc. Generally, the optical pick-up includes a laser diode used as a light source, an optical system for guiding a laser light beam into an optical disc, an objective lens for converging the laser light beam onto the optical disc in a spot shape and an actuator for moving the objective lens in the focusing direction of optical disc or in the width direction of track. The actuator for optical pick-up allows the light spot to trace tracks in an optical recording medium even though the recording medium occurs a surface vibration, an eccentricity and so on by driving the objective lens around two axes.
As shown in FIG. 1, such an optical pick-up actuator includes an objective lens 2, a lens holder 4, permanent magnets 6, yokes 8, tracking coils 10, a focusing coil 12 and a wire spring 14. The objective lens 2 converges a laser light beam generated at the laser diode onto the information recording surface of the optical disc in a spot shape. The lens holder 4 inserts and secures the objective lens 2 to support the objective lens 2. To this end, the center of the lens holder 4 is provided with a circular hole for securing the objective lens 2. The focusing coil 12 is wound around the circumference surface of the lens holder 4. The tracking coils 10 are attached to the focusing coil 12 to thereby form a closed current loop perpendicular to a current flowing the focusing coil 12. The permanent magnets 6 are arranged one by one at the upper portion and the lower portion of the lens holder 4 in such a manner to be opposite to the tracking coil 10. Each permanent magnet 6 is attached to the yoke 8 made from a magnetic material such as still and the like so as to guide a magnetic flux. The permanent magnets 6, the tracking coils 10 and the focusing coil 12 form a magnetic circuit generating a Lorentz force under the Fleming""s left-hand rule. The objective lens 2 is moved in the focusing direction or the tracking direction by means of the Lorentz force generated at the magnetic circuit. The wire spring 14 forms a current path between the tracking coil 10 and the focusing coil 12 and, at the same time, acts as a driving axis of the lens holder 4 when the lens holder 4 is moved in the focusing direction(i.e., upward or downward direction) or the tracking direction(i.e., left or right direction). Also, the wire spring 14 serves to support the lens holder 4 by means of its elastic force.
The optical pick-up actuator generates Lorentz forces by means of the tracking coils 10 and the focusing coil 12 arranged within a magnetic space formed with the permanent magnets 6 and the yokes 8. The objective lens 2 is moved in the upward, downward, left, or right direction along with the lens holder 4 by the Lorentz forces to thereby control a focusing and a tracking. As shown in FIG. 2A, a magnetic circuit for the focusing consists of a focusing coil 12 and a permanent magnet 6 magnetized into N pole or S pole. A magnetic flux generated at the permanent magnet 6 passes through the permanent magnet 6 and the yoke 8 to interlink the focusing coil 12. In this case, the Lorentz force emerges in the vertical direction by a current applied to the focusing coil 12, the horizontal surface of which is perpendicular to the magnetic flux. The objective lens 2 is moved in the vertical direction(i.e., upward or downward direction) along with the lens holder 4 by means of the Lorentz force, thereby controlling a size of the light spot on the optical recording medium. As shown in FIG. 2B, a magnetic circuit for the tracking consists of a permanent magnet 6 magnetized into N and S poles, and tracking coils 10 opposed to the magnetic surface of the permanent magnet 6. In this case, the Lorentz force emerges in the horizontal direction by a current applied to the tracking coil 10, the vertical surface of which is perpendicular to the magnetic flux. The objective lens 2 is moved in the horizontal direction(i.e., left or right direction) along with the lens holder 4 by the Lorentz force, thereby moving a light beam on the optical recording medium in the track direction. The permanent magnet 6 used in such a magnetic circuit is magnetized in such a manner that magnetic flux lines progress in parallel within itself. Since the magnetic flux lines progress in parallel within the permanent magnet 6, as shown in FIG. 3, only magnetic flux lines generated at the center of the permanent magnet 6 are interlinked with the tracking coil 10 or the focusing coil 12. In other words, many magnetic flux lines generated at the upper side and the lower side of the permanent magnet 6 are leaked. This leakage of magnetic flux lines causes a deterioration in the efficiency, that is, the sensitivity of the permanent magnet 6. According to an experimental data in this regard, a magnetic flux density interlinked with the tracking coil or the focusing coil 12 is about 2000 to 3000 Gauss and a sensitivity in a frequency of 200 Hz is about 0.035 to 0.045 mm/V in the focusing direction and about 0.020 to 0.030 mm/V in the tracking direction in the case of the magnetic circuits as shown in FIG. 2A and FIG. 2B.
The optical pick-up actuator is preferable to have a great actuation amount with respect to a small signal because an allowance range in a size of the optical spot converged onto the optical disc becomes smaller as the optical disc has a higher density. Also, the optical pick-up actuator requires a wider servo band and a higher acceleration as a recording/reproducing apparatus has a high-multiple speed. In other words, the optical pick-up actuator is required to have a high sensitivity at a wide band including both the low frequency band and the high frequency band.
The sensitivity of the optical pick-up actuator can have a different value depending upon a magnetic flux density, an effective length of coil, a weight of actuating part and so on. Particularly, since a sensitivity in both the low frequency band and the high frequency band is considerably enhanced when a magnetic flux density of the permanent magnet 6 becomes high, a scheme for improving the permanent magnet has been raised as an important factor for making a high sensitivity of optical pick-up actuator. A high grade of neodymium sintered magnet capable of generating many magnetic flux lines is used as the permanent magnet to satisfy such a high sensitivity characteristic. This neodymium sintered magnet is an expensive material in itself. Furthermore, a higher grade of neodymium sintered magnet causes a higher rise in the material cost. Accordingly, it becomes necessary to enhance the efficiency of permanent magnet and the driving sensitivity in the optical pick-up actuator with a view to keeping up with a trend toward the high density and the high-multiple speed.
Accordingly, it is an object of the present invention to provide an actuator for optical pick-up using concentrated magnetization method wherein a driving sensitivity is improved in a wide frequency band.
In order to achieve these and other objects of the invention, an actuator for optical pick-up using the concentrated magnetization method according to one aspect of the present invention includes a lens holder attached to an objective lens; a coil wounded around the lens holder to receive a current; and permanent magnet means for constructing a magnetic circuit along with the coil so as to move the objective lens, the magnet means being magnetized in such a manner to concentrate a magnetic flux on the effective surface of the coil.
An actuator for optical pick-up using the concentrated magnetization method according to another aspect of the present invention includes a lens holder attached to an objective lens; a tracking coil wounded around the lens holder; a focusing coil adhered to the tracking coil to receive a current in a direction perpendicular to a direction of a current applied to the tracking coil; permanent magnet means for constructing a magnetic circuit along with the coils, the magnet means being magnetized in such a manner to concentrate a magnetic flux on the effective surfaces of the tracking coil and the focusing coil; a yoke for guiding a magnetic flux generated at the permanent magnet means into the tracking coil and the focusing coil; a frame for supporting the yoke and the permanent magnet means; and an elastic member for delivering currents applied to the tracking coil and the focusing coil and for stabbly supporting the lens holder upon movement of the objective lens.